The present invention is concerned with mandrels for producing electroformed shapes suitable for use as anode material in titanium baskets for electroplating. Such shapes, in particular, nickel Rounds (TradeMark INCO, Ltd.) have been produced in the past on mandrels which have a base made of a stainless steel sheet and an insulating coating, such as an epoxy enamel. This insulating coating covers the entire surface of the mandrel except for open areas in the desired shape of the electroformed pieces. Typically, such a mandrel will be a stainless steel sheet, one meter square, possibly 1 to 5 mm thick and having a multiplicity of circular open areas in the epoxy coating each having diameters of about 19 to about 32 mm. The difficulty with these mandrels is that they will be satisfactory for only a relatively short period of time, such as ten (10) electroforming cycles. At the end of their useful life, the mandrels must be stripped of the epoxy coating and refinished. This takes substantial time and is costly.
In U.S. application Ser. No. 864,274, filed Dec. 27, 1977, now U.S. Pat. No. 4,158,612, Messrs. Luch and Chart described a polymeric mandrel for electroforming, which consists essentially of an electroconductive base, an electroconductive polymeric sheet superimposed on that base and a masking layer of non-electroconductive polymeric material overlaying the electroconductive polymeric layer. The masking layer has a number of holes cut therethrough in order to provide access of electrolyte to the electroconductive layer and to establish the shape of the items to be electroformed. Again typically, the electroformed items are discs and the cutouts in the masking layer are circular. U.S. Pat. No. 4,040,937 of record in the file of U.S. application Ser. No. 864,274, is concerned with an electroconductive polymeric mandrel but discloses use of this mandrel for forming starting sheets. There is no disclosure or suggestion in this patent of any mandrel which will produce items having a maximum of a major dimension of about 50 mm. A difficulty with the mandrel of U.S. application Ser. No. 864,274 is that, in use, the electrodeposited material on the electroconductive polymer layer tends to enlarge the area exposed to electrolyte. Although the joint between conductive and non-conductive material is designed to be a hermetically sealed joint, in that, there is a continuity of polymer in both the electroconductive and non-conductive layer, it has been found that the lateral growth of metal during electrodeposition can cause enormous pressures and can deform the shape and size of the openings in the masking layer despite the excellent bond existing between the masking layer and the electroconductive layer.